1. Field of the Invention
The present invention relates to an optical fiber amplifier for receiving pumping light and signal light, amplifying the signal light, and outputting thus amplified signal light; and an optical fiber for optical amplification employed in this optical fiber amplifier. In particular, the present invention relates to an optical fiber for optical amplifier in which at least a part of its core is doped with erbium, and an optical fiber amplifier using this optical fiber.
2. Related Background Art
As a technique for increasing the amount of transmittable information, i.e., channel capacity, in optical communications networks, wavelength division multiplexing (WDM) communications technology has been known.
In the WDM communications, there has been a demand for developing a super-wide band optical fiber amplifier which can simultaneously amplify WDM signal lights. An example of such a technology is disclosed in Makoto Yamada et al., xe2x80x9cSuper-wide Band Optical Fiber Technology Capable of Simultaneously Amplifying Communication Wavelength Bandxe2x80x9d (NTT Technical Journal, November issue, 1998, pp. 76-81). This publication discloses an optical amplifier in which an optical fiber amplifier section using an Er-doped optical fiber having an amplification region in the wavelength band of 1.55 xcexcm and an optical fiber amplifier section using an Er-doped optical fiber having an amplification region in the wavelength band of 1.58 xcexcm are connected in parallel, thereby yielding a wide flat gain region over both 1.55 xcexcm and 1.58 xcexcm bands.
However, the amplification factor per unit length (gain per unit length) of the Er-doped optical fiber conventionally employed in the Er-doped optical fiber amplifier in the 1.58 xcexcm band is smaller than that in the 1.55 xcexcm band by at least one order of magnitude. Therefore, for obtaining a signal gain equivalent to that in the 1.55 xcexcm band, it is necessary for the optical fiber for amplifying the 1.58 xcexcm band to have a length at least 10 times that for the 1.55 xcexcm band. Thus it reaches several hundred meters. It is disadvantageous in that the apparatus increases its size.
For improving the gain per unit length of an optical fiber for amplification, a technique for increasing the concentration of Er so as to enhance the absorption of pumping light by Er may be considered. However, increasing the Er concentration causes the concentration quenching, thereby lowering energy conversion efficiency. A concentration of about 1000 wt-ppm has conventionally been considered a limit for Er ion concentration. It makes thus difficult to improve the gain per unit length.
In view of the above-mentioned problems, it is an object of the present invention to provide an optical fiber for optical amplification having a high gain per unit length in the 1.58 xcexcm band, and an optical fiber amplifier using the same.
As a result of diligent studies for overcoming the above-mentioned problems, the inventors have found that suppressing the association of Er ions by co-doping of Al restrains the concentration quenching. This indicates increasing the Er concentration greater than conventional fiber. Further, as a technique for increasing the amount of Er absorption, enhancing the overlap between the Er-doped area and optical power distribution is considered. For realizing this, the inventors have found it effective to shift the cutoff wavelength to the longer wavelength side. It enhances the light confinement in the core.
Namely, shifting the cutoff wavelength to the longer wavelength side enhances the light confinement in the core, thereby increasing the absorption by Er. Also, the limit for concentration of doped Er where the concentration quenching occurs shifts to the higher concentration region, whereby the amount of Er ions can be increased. As increasing the concentration of doped Er and shifting the cutoff wavelength to the longer wavelength side, an optical fiber for optical amplification having a high gain per unit length can be obtained. In addition, it is also effective in that bending loss decreases, whereby the optical fiber for optical amplification can be wound into a small coil, so as to be packaged.
The present invention is based on the above-mentioned finding. The optical fiber for optical amplification in accordance with the present invention is an optical fiber for optical amplification used for 1.58 xcexcm band signal amplification, at least a core region thereof being doped with Er, wherein at least a part of the core region is made of silica glass co-doped with Ge and Al together with Er whose average atomic concentration in the core region is from 1000 wt-ppm to 3000 wt-ppm inclusive, and cutoff wavelength is from 1.3 xcexcm to 1.5 xcexcm inclusive.
Thus shifting the cutoff wavelength to the longer wavelength side enables the core to have an improved optical power confinement coefficient of 0.8 or higher, whereas doped Er of 1000 wt-ppm or more allows the optical amplification in the 1.58 xcexcm band to yield a gain per unit length on a par with that in the 1.55 xcexcm band.
On the other hand, the optical fiber amplifier of the present invention comprises the above-mentioned fiber for optical amplification, a pumping light source which emits pumping light in a 1.48-xcexcm band for exciting the optical fiber for optical amplification, a multiplexer/demultiplexer which introduces the pumping light emitted from the pumping light source into the optical fiber for optical amplification on which the signal lights are incident, and respective optical isolators disposed at input and output ends of the signal lights.
Preferably, the mode field diameter (MFD) of the optical fiber for optical amplification in the 1.58 xcexcm band is 5 xcexcm or more.
Setting the MFD to at least 5 xcexcm is preferable for lowering the power density of signal light. Thereby it reduces nonlinear effects such as distortions in signal waveform caused by self phase modulation and crosstalk of signals caused by four-wave mixing. It is also effective in lowering the splicing loss when it is connected to fibers for signal transmission, thereby becoming advantageous for improving energy conversion efficiency.
The Al concentration in the core region of the optical fiber for optical amplification is preferably 1 wt % or more. And it is more preferably from 5 wt % to 20 wt % inclusive. When the Al concentration is thus set, the association of Er ions would be suppressed, whereby the concentration quenching limit can be shifted to the higher concentration side.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.